The present invention relates to a high-transverse-curvature tire, in particular for motor-vehicles.
The invention is more particularly addressed to a front tire for two-wheeled vehicles in which the high transverse curvature is defined by particular values of the ratio between the height of the tread crown from a line passing through the tread axial width extremities, measured at the equatorial plane, and the distance between said tread extremities. This value, in any case at least as high as 0.3, is higher than that concerning the corresponding rear tire, which usually does not exceed 0.3.
In the following of the present description said ratio will be referred to as "curvature ratio" or, in a usual manner, as "tread camber".
It is known that a two-wheeled vehicle while travelling over a curvilinear path tilts on the side internal to the curve at an angle defined as the camber angle, which angle can reach values as high as 65.degree. relative to a plane which is vertical to the plane of the ground; this operation brings the tires to a point to cause a slip thrust counteracting the centrifugal force acting on the vehicle.
It is also known that as the vehicle speed gradually increases and the radius of curvature of the path of travel gradually decreases, the tire tilting alone is not sufficient to compensate for the effect of the centrifugal force: therefore an increase in the slip thrust is required and this is achieved by tilting the rolling plane of the front tire relative to the tangential direction of the curvilinear path according to an angle referred to as "drift angle", by an operation carried out by the driver through steering the handlebar. In this manner the necessary increase in the slip thrust is achieved. Of course, as is well known, the rear tire cannot be steered.
In order to be able to create this greater slip thrust necessary to balance the centrifugal force effect, the front tire includes a belt, disposed between the carcass and the tread, said belt being formed of at least two radially superposed strips of cords symmetrically inclined to the equatorial plane of the tire.
This belt structure has a high stiffness, useful to generate appropriate thrust slips, which stiffness is however too great from the point of view of comfort, because the handlebar may only be controlled with a very strong effort, which will bring about weariness of the driver, and vibrations resulting from the road-bed unevenness are transmitted to the handlebar.
One could think of replacing the above mentioned belt made of strips of crossed cords with a more flexible belt structure provided with longitudinally-oriented cords which is capable of better taking up the ground unevenness. A tire of this type is described for example in the European Patent Application EP 0 565 339 A1; said tire is provided with a belt, between the carcass and tread, comprising at least one layer of zero-degree cords distributed in a configuration offering a greater thickness at the sides than in the middle.
More particularly, the cord amount in a given central area has a value of between 60% and 80% of the amount of cords close to the tire shoulders.
However, this solution is not feasible for many reasons; first of all because such a tire is not capable of developing important slip thrusts. In fact, the zero-degree cord layer does not offer a sufficient strength to the tangential forces acting in a direction transverse to the cord lying. Secondly, due to the substantial inextensibility of the cords, the zero-degree winding up must be carried out on a previously shaped support, in particular an auxiliary manufacturing drum suitably made convex, such as for example the drum described in U.S. Pat. No. 5,437,006 filed by the present Assignee, or directly on the previously shaped carcass.
However, making a zero-degree cord winding on said drum involves increasingly greater difficulties as the curvature increases, which difficulties can be overcome only with the aid of drums having a low transverse curvature and by virtue of the presence of an underlying rubberized layer promoting adhesion of the cord coils and preventing the wound end coils on the drum surface from slipping off.
In this case, it is pointed out that the rubberized layer has only the function of ensuring, by its adhesive character, the geometrical and structural stability of the layer of coils being formed as a result of winding of the cord on the drum, so that in the finished tire said rubberized layer is only a nuisance element increasing the tire cost and weight (which weight increase exactly occurs in the region where the centrifugal force takes the highest values) without involving any structural or behavioral advantage.
One could then think of making said tire with a belt comprising a layer of zero-degree cords and two layers of cords symmetrically inclined to the equatorial plane of the tire and also having the function of enabling winding of the coils around said drum; it is well apparent that this belt could not solve the problem coped with in the present invention, as it would have even more stiffness than the value requiring to be reduced.
For the above reasons, the usual tires for front wheels for motor-cycles generally have belt structures consisting of two superposed strips having crossed cords.
Such a fact implies another drawback to be added to the already mentioned great stiffness, which on the other hand is necessary. The ideal condition that one would like to reach in such a belt is that involving a low stiffness at the central tread portion which is engaged when the vehicle is running on a straight stretch, in which condition flexibility is more useful than stiffness, and a high stiffness at the tread extremities which are engaged when the vehicle is running on a curve, in which condition strong slip thrusts are necessary.
This differentiated stiffness could be achieved by grading the thickness values of the cords in the corresponding belt portions, but in practice this feature has not yet been achieved.
According to a first known manufacturing method, the two belt strips with crossed cords are disposed in a flat arrangement on the shaped carcass and the side portions of the strips are bent down onto the carcass sidewalls. Thus a greater thickness of the cords on the sidewalls is achieved, according to wishes. Unfortunately, this operation involves a very important deformation of the strips which must conform to the sidewall surface of the carcass of decreasing diameter and therefore causes a strong structural unevenness in the carcass of the finished tire.
According to another known manufacturing method, the belt is disposed in a flat arrangement on the carcass in the form of a cylindrical sleeve or on the already mentioned auxiliary manufacturing drum and the assembly is then expanded until a toric form is reached; the operation involves a reduction in the crossing angles between the strip cords and in the cord angle of each strip relative to the equatorial plane of the tire close to said plane, which will result in a greater thickness of the cords in the central area and lower thickness in the side areas of the belt.
Practically a cord thinning out is achieved where a greater thickness should be required and vice versa, so that the stiffness values are reversed with respect to the required optimal values.
In spite of the above, the last-mentioned manufacturing method is preferred for use, just to avoid the inconvenience of the structural unevenness encountered with the first manufacturing method.
The use of belt strips with zero-degree cords however has been already proposed in different publications.
U.S. Pat. No. 5,301,730 discloses a tire comprising a toric carcass, a tread band and a belt between the carcass and tread, in which the belt is formed of three cord layers and more particularly: a radially innermost layer with inextensible zero-degree cords, two layers radially external to the zero-degree layer and comprising cords crossed with each other according to given angles symmetrical to the equatorial plane.
It is apparent that the above tire represents the worst possible solution for achievement of the intended purposes: the accomplishment of a cord arrangement at an angle of zero degrees on the radially innermost outline requires the use of a rubberized layer to be interposed between the drum and the zero-degree winding, which will bring about many drawbacks.
On the other hand, due to its radially internal position, it is inappropriate to control the effects of the centrifugal stresses on the radially outermost layers having crossed cords, as these layers under the effect of said centrifugal forces can expand and thereby impair the belt stability and the integrity of the tire itself. Also known from German Patent DE 34 10 857 is a tire for motor-bikes comprising a radial carcass, a tread and a three-layered belt interposed between the carcass and tread. The belt is comprised of two layers of cords crossed with each other respectively at angles of 56.degree. and 135.degree. to the equatorial plane and a radially outermost layer of cords at an angle of zero degrees.
This solution has been already discussed beforehand and the greater stiffness of same relative to the known belts devoid of the zero-degree cord layer has already been emphasized.